Detail Synthesis Route of Free-Standing Antiferromagnetic La2CuO4 Nanoparticles via Sol-gel Method

We report the detailed synthesis route of free-standing antiferromagnetic La2CuO4 nanoparticles by the sol-gel method. The precursors were needed to be dissolved in acid solution and continued with heating treatment to make a gel. Some catalyst was needed to control and stabilized the transformation from the solution to gel. Some heating treatments were also conducted to remove the organic component and for the nucleation process to obtain the single-phase sample. Although the sol-gel method required some additional precursors such as acid and catalyst, it has some advantages. It needed a relatively low sintering temperature and short sintering time compare to those at the solid-state reaction. Interestingly, this method can produce a sample with a good distribution of the particle size.


Introduction
Nano-sized effects have attracted great attention recently since it leads to the alter of physical and magnetic properties of a material. One of the interesting phenomena is the appearance of magnetism in nano-system such as nanogold and nano-CuO [1][2][3][4][5]. Clear evidence of magnetism was observed in gold nanoparticles encapsulated by butanethiol with Au-cluster size at 2.2 nm by muon-spin relaxation (μSR) [2]. From zero-field, ZF-μSR time spectra, fast spin relaxation was observed with decreasing temperature indicating the slowing down of electronic magnetic moment fluctuation [2]. The existence of a fluctuating electronic magnetic moment was also confirmed from the longitudinal field, LF-μSR, in which the slowly relaxation behavior still survives by applying a field of 2 kG [2]. That study could not distinguish whether the magnetic moment is located at the encapsulated molecule or inside the gold nanoparticles. In the case of an antiferromagnetic, CuO, hysteresis loop was also observed when the particle size of about 6.6 nm at 10 K indicating the presence of magnetism [6]. Furthermore, the uncompensated spins at the surface of nanoparticles were proposed to explain the reason behind the appearance of magnetism in CuO nanoparticles.
The suggestion about uncompensated spins at the surface was proposed leading to magnetism in other antiferromagnetic systems such as La2CuO4. The ferromagnetic component was observed in La2CuO4 at 20 K until 350 K when its particle size was reduced to 1.53 µm [7]. Both CuO nanoparticles and La2CuO4 microparticles were not encapsulated by other molecules, but the explanation on these systems is limited to the role of the surface effect only. In the nano-sized effects, there is one more important role. That is the quantum size effect (QSE) which was predicted by Kubo in 1962 [8]. Kubo determined the average of electron level spacing, δ, to be inversely proportional to the density of states, D(εF), at the Fermi energy, εF [8]. The physical properties of metallic nanoparticles would significantly differ from the bulk because the energy level of conduction electrons is predicted to become discrete [8,9], as shown in Eq. 1: where ℏ is Plank's constant, m* is the effective mass of an electron, n is the electron density, and V is the average volume of the nanoparticles. This prediction is based on the consideration that the electronic state is redistributed near εF when the particle size of a system is reduced down to nanometer size [8,9]. Even though this theory is believed to happen in nanoparticles, the identification of QSE is still difficult to be observed. This is because when the particle size is reduced to be in nanometer-size, the surface effect will be dominant which also shows the deviation of the physical properties of nanoparticles from the bulk properties [9]. Therefore, it is a challenging part to clarify whether the change of the properties of a system is due to QSE or surface effect.
Among the systems mentioned above, we aim to investigate nano-sized effects in La2CuO4. The problem that we need to tackle in this system is how to control its particle size within nanometer-size. Some synthesis parameters are needed to be considered to make a single phase of La2CuO4 nanoparticles. Since the detailed and systematic synthesis route has not been reported yet, we report our detailed synthesis route to produce La2CuO4 nanoparticles.

Experimental Methods
The samples were prepared by using a chemical reaction method which is believed to be one of the best methods to produce nanoparticles [10]. Chemical reaction method is a bottom-up method which means spontaneous self-assembly from the atomic level to the nano-scale level. There are some techniques in the chemical reaction method, such as chemical vapor deposition and sol-gel. We chose sol-gel in this study because a sample with a more homogeneous distribution of the particle size could be obtained through this method [11]. Besides that, it needs a relatively low heating treatment.
La2O3 (Merck, 99.5%) and CuO (Merck, 99.5%) were used as precursors or starting materials. Nitric acid solution (Merck, 69%) was used to dissolve La2O3 and CuO to make them become La(NO3)3.6H2O and Cu(NO3)2, respectively. Citric acid monohydrate, C6H8O7.H2O (Merck, 99.5%) is a weak acid that was used as a chelating agent to control the transformation from solution to be gel so that the uniform gel can be achieved. Ethylene glycol, (C2H6O2, Merck 99.5%) was used as a stabilizing agent in the sol-gel process. Besides that, citric acid monohydrate and ethylene glycol lead to fasten the bonding reaction among La 3+ , Cu 2+ , and O 2-. The ratio of CA and EG was 3:2. The role of CA as chelating agent and to fasten the bonding reaction will be active at a certain pH value. The dissociation reactions of CA are shown as Eq. 2 until Eq. 7 [12]. Ammonia solution (Merck, 25%) was used to adjust the pH of the solution.
H3Cit ↔ H2Cit -+ H + (Kα1 = 7.4 × 10 -4 ) (2) The detailed synthesis route and the sample condition in each step are explained in the next section. X-ray diffraction (XRD) measurement with Cu-Kα radiation at room temperature was performed at the Department of Engineering and Applied Sciences, Sophia University, to check the purity of the samples. The XRD pattern was refined by the Rietveld analysis by using General Structure and Analysis Software (GSAS-II).

Results and Discussion
As the first step, we need to pre-fire the La2O3 at 700 o C for two hours since it is sensitive to the air and water. After that, measure the weight of La2O3 and CuO based on the stoichiometric calculation. Then, dissolved each precursors in the nitric acid solution until a transparent solution was achieved as shown in Fig. 1a and b. After that, mix all of them, then add citric acid monohydrate and ethylene glycol to the solution.
The stirring process was done until the transparent solution was achieved, as shown in Fig. 1c. At this stage, the pH of the solution is about 2, which indicates that the citric acid exists as H3Cit and H2Cit -. It will be dissociated to be HCit 2-and Cit 3-when we increase the pH of the solution by adding ammonia solution. We have to add ammonia solution little by little since the temperature of the solution will be automatically increased due to the presence of the ammonia solution. Keep adding the ammonia solution to increase the pH value of the solution. After some trial and error, we got that a pH of 7 is the best condition to make a single phase of La2CuO4 nanoparticles. Figure 2 shows the solution after achieving a pH of 7.
The next step is to make the solution become gel by setting the temperature of the magnetic stirrer to be around 80 o C and keeping stirring the solution. The gel, as shown in Fig. 3 will be obtained at this stage. Heat the gel at 150 o C to remove all water components from the solution until the dried blue gel was formed, as depicted in Fig. 4. Then, heat it at 300 o C for 3 hours to remove all organic components. This is the socalled calcination process. There was a volume expansion of the sample forming the sponge-like product, as shown in Fig. 5. Grind the spongelike product to be in a powder form as shown in Fig. 6. Furthermore, the sintering process of the powder for the nucleation process so that the phase of La2CuO4 will be formed. The minimum sintering condition to make a single phase of La2CuO4 is 600 o C for 45 minutes [13]. It is lower than that of the solid-state reaction which needs the sintering temperature of above 1000 o C for around 16 hours. After the sintering process, grind the sample to make fine particles. The final product is powder, as depicted in Fig. 7. The heating condition of pre-fire process, calcination, and sintering are shown in Figure 8a, b, and c, respectively. Figure 9 shows the XRD pattern of the La2CuO4 nanoparticles. The sample is a single-phase with no impurity phase detected. It means the synthesis route and synthesis parameters used here are in good condition to make La2CuO4 nanoparticles. Rietveld refinement results have been shown in our former paper showing that the system has orthorhombic crystal structure with Bmab space group which is as same as those observed in the bulk case [13]. The particle size of the sample is determined by using TEM showing that the higher sintering temperature and the longer sintering time, the bigger particle size was obtained [13,14].

Conclusions
The preparation of La2CuO4 nanoparticles was successfully conducted by using the sol-gel method. Some synthesis parameters were needed to be controlled, such as the stoichiometric mass of each precursor, the pH of the solution, the mass ratio between catalysts, sintering time, and sintering temperature. In this work, the La2CuO4 exhibited a single-phase without impurity phase in nanometric size.